Interfacial electron transfer dynamics of two newly synthesized catecholate bound RuII polypyridyl-based sensitizers on TiO2 nanoparticle surface - A femtosecond pump probe spectroscopic study

Article abstract of DOI:10.1002/ejic.201100411

Two new catecholate-bound Ru^II-polypyridine based sensitizers, (2,2′-bipyridine){ethyl 3-(4-hydroxyphenyl)-2-[(4′-methyl-2, 2′-bipyridinyl-4-carbonyl)amino]propionate}{4-[2-(4′-methyl-2, 2′-bipyridinyl-4-yl)vinyl]benzene-1,2-diol)}ruthenium(II) hexafluorophosphate (5) and [(2,2′-bipyridine)-(4-2,2′-bipyridinyl- 4-yl-phenol)-(4-{2-(4′-methyl-2,2′-bipyridinyl-4-yl)vinyl}benzene-1, 2-diol)]ruthenium(II) hexafluorophosphate (6) with secondary electron-donating groups (tyrosine and phenol, respectively) were synthesized and characterized. Steady-state optical absorption and emission studies confirm strong coupling between the sensitizers and TiO₂ nanoparticles. Femtosecond visible transient absorption spectroscopy has been employed to study interfacial electron transfer (IET) dynamics in the dye-nanoparticle systems to explore the influence of the secondary electron-donating groups on IET dynamics. Electron injection into the conduction band of nanoparticulate TiO₂ has been confirmed by detection of the conduction band electrons in TiO₂ ([e^-]_TiO2^CB) and radical cation of the adsorbed dye (D^·+) in real time monitored by transient absorption spectroscopy. A single exponential and pulse-width limited (< 100 fs) electron injection has been observed. Back electron transfer (BET) dynamics have been studied by monitoring the decay kinetics of the injected electron in the conduction band of TiO₂ and by the recovery of the ground state bleach. BET dynamics in dye-TiO₂ systems for complexes 5 and 6 have been compared with those of [bis(2,2′-bpy)-(4-{2-(4′-methyl-2, 2′-bipyridinyl-4-yl)vinyl}benzene-1,2-diol)]ruthenium(II) hexafluorophosphate (7), which does not have a secondary electron-donating group.

Full text of DOI:10.1002/ejic.201100411

FULL PAPER
DOI: 10.1002/ejic.201100411
Interfacial Electron Transfer Dynamics of Two Newly Synthesized Catecholate
Bound RuII Polypyridyl-Based Sensitizers on TiO2 Nanoparticle Surface –
A Femtosecond Pump Probe Spectroscopic Study
Tanmay Banerjee,[b] Sachin Rawalekar,[a] Amitava Das,*[b] and Hirendra N. Ghosh*[a]
Keywords: Ruthenium / Nanoparticles / Electron transfer / Electron-donating group / Time-resolved spectroscopy
Two new catecholate-bound RuII–polypyridine based sensi-
tizers, (2,2Ј-bipyridine){ethyl 3-(4-hydroxyphenyl)-2-[(4Ј-
methyl-2,2Ј-bipyridinyl-4-carbonyl)amino]propionate}{4-[2-
(4Ј-methyl-2,2Ј-bipyridinyl-4-yl)vinyl]benzene-1,2-
diol)}ruthenium(II) hexafluorophosphate (5) and [(2,2Ј-bipyr-
idine)-(4-2,2Ј-bipyridinyl-4-yl-phenol)-(4-{2-(4Ј-methyl-2,2Ј-
bipyridinyl-4-yl)vinyl}benzene-1,2-diol)]ruthenium(II) hexa-
fluorophosphate (6) with secondary electron-donating groups
(tyrosine and phenol, respectively) were synthesized and
characterized. Steady-state optical absorption and emission
studies confirm strong coupling between the sensitizers and
TiO2 nanoparticles. Femtosecond visible transient absorption
spectroscopy has been employed to study interfacial electron
transfer (IET) dynamics in the dye–nanoparticle systems to
explore the influence of the secondary electron-donating
groups on IET dynamics. Electron injection into the conduc-
tion band of nanoparticulate TiO2 has been confirmed by de-
CB
tection of the conduction band electrons in TiO2 ([e–]TiO2
)
and radical cation of the adsorbed dye (D·+) in real time mon-
itored by transient absorption spectroscopy. A single ex-
ponential and pulse-width limited (Ͻ 100 fs) electron injec-
tion has been observed. Back electron transfer (BET) dynam-
ics have been studied by monitoring the decay kinetics of the
injected electron in the conduction band of TiO2 and by the
recovery of the ground state bleach. BET dynamics in dye–
TiO2 systems for complexes 5 and 6 have been compared
with those of [bis(2,2Ј-bpy)-(4-{2-(4Ј-methyl-2,2Ј-bipyridinyl-
4-yl)vinyl}benzene-1,2-diol)]ruthenium(II) hexafluorophos-
phate (7), which does not have a secondary electron-donat-
ing group.
tor interface, and a long lived charge separated state is im-
perative in order to obtain a good photocurrent response.
Longer lived charge separated states may be achieved by
making the positive charge move away from the photo-oxi-
dized sensitizer core, which is possible by covalently at-
taching an electron donor to the sensitizer. By connecting
phenothiazine or triarylamine to RuII–polypyridine-based
complexes, long lived charge separation has been ob-
tained.[19–29] Much work has been carried out on systems
where a secondary redox couple based on phenol has been
used to rereduce the RuIII created by excited state electron
transfer to nanoparticulate TiO2 analogous to electron
transfer from Tyrz to P 680 in photosystem II.[30,31] How-
ever, most of this work has been carried out with sensitizers
based on carboxylate anchoring groups, which have certain
disadvantages. Due to the low pKa of carboxylates (pKa
≈ 3.5)[32,33] there remains a possibility of slow desorption of
the sensitizer dye molecules from the surface of semicon-
ductor in the presence of water. Furthermore, dyes with
carboxylate anchoring groups generally show biphasic elec-
tron injection.[17,34] Our previous reports show that ruthe-
nium–polypyridyl-based sensitizer dyes with catecholate an-
choring groups inject electrons into TiO2 single exponen-
tially and within instrumental pulse width (Ͻ 100 fs)[35–37]
in contrast to dyes with carboxylate anchoring groups. In
addition, catecholates have pKa values greater than 9.6 and,
Introduction
Research in the area of nanocrystalline dye-sensitized so-
lar cells (DSSC) has attracted considerable interest owing
to their potential applications as cost-effective alternatives
to current p–n junction photovoltaic devices.[1–4] The stud-
ies in this area have intensified over the past decade with
an aim to improve the overall conversion efficiency (η) from
10.4%, a value achieved by Grätzel and coworkers[2] in N3-
sensitized TiO2 DSSC.[5–10] Over the years efficient dye-sen-
sitized solar cells based on RuII–polypyridine complexes at-
tached to nanocrystalline TiO2 have been developed.[4,11–18]
RuII–polypyridine complexes have been the choice because
of their prominent visible absorption bands, long-lived ex-
cited states and their distinctive photochemical stability.
One of the major factors controlling the overall light-to-
current conversion yield in such light-harvesting devices is
the effective charge separation at the sensitizer–semiconduc-
[a] Radiation and Photo Chemistry Division, Bhabha Atomic
Research Center,
Mumbai, India
Fax: +91-22-25505151
E-mail: hnghosh@barc.gov.in
[b] Analytical Science Discipline, Central Salt and Marine
Chemicals Research Institute,
Bhavnagar, Gujarat, India
Fax: +91-278-2567562
E-mail: amitava@csmcri.org
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T. Banerjee, S. Rawalekar, A. Das, H. N. Ghosh
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even at higher pH, the possibility of the dye molecules be-
coming desorbed from the surface of the semiconductor is
excluded. Moreover, studies concerning the effects of the
electron donor have been limited to longer time domains
(micro- and millisecond) when a good fraction of charge
separated species have already undergone a charge recombi-
nation reaction. As a result it is difficult to understand the
effect of the secondary electron-donating group in IET dy-
namics. To understand the mechanism and the effect, it is
very important to study IET dynamics on the ultrafast
timescale.
With these aspects in mind we have synthesized two new
RuII–polypyridyl-based sensitizer molecules,
5 and 6
(Scheme 1), with different electron-donating abilities of the
secondary groups (tyrosine and phenol, respectively) and a
catecholate anchoring group to bind to TiO2 nanoparticles.
Steady state optical absorption and emission studies con-
firm strong binding of the sensitizer molecules to TiO2. To
understand the effect of secondary electron-donating
groups on the ultrafast timescale, we have studied the IET
dynamics in 5- and 6-sensitized TiO2 nanoparticles using
femtosecond transient absorption spectroscopy and the re-
sults have been compared with our previous results for 7-
sensitized TiO2 nanoparticles.[37]
Scheme 1. Molecular structures of 5, 6 and 7.
Scheme 2. Synthetic route for the preparation of 5 and 6.
Results and Discussion
column chromatography yielded pure 2. Complex 5 was
prepared in a one pot reaction where RuCl3·xH2O was al-
Synthesis
The synthetic methodology adopted for the synthesis of lowed to react successively with molar equivalents of 2,2Ј-
the bipyridine-based ligands and complexes are outlined in bipyridine, 2 and 4 at different time intervals at progress-
Scheme 2. 4-(2,2Ј-Bipyridinyl-4-yl)phenol and 4-[2-(4Ј- ively higher temperatures. After evaporation of the solvent,
methyl-2,2Ј-bipyridinyl-4-yl)vinyl]benzene-1,2-diol
were anion exchange was carried out in acetonitrile by stirring
synthesized as per previous reports and the analytical data in the presence of excess potassium hexafluorophosphate.
matched well with that of the reported compounds. Com- Acetonitrile was used because the product was only par-
pound 1 was prepared by a selenium dioxide oxidation of tially soluble in water. The crude product was purified by
4,4Ј-dimethyl-2,2Ј-bipyridine followed by further oxidation column chromatography and was further purified by vap-
with silver nitrate. The excess silver nitrate was precipitated our diffusion method of recrystallization. Complex 6 was
as silver oxide and was removed by filtration. Adjusting the prepared following a similar methodology. The addition of
pH of the aqueous extract to about 3.5 precipitated the 4-[2-(4Ј-methyl-2,2Ј-bipyridinyl-4-yl)vinyl]benzene-1,2-diol
carboxylic acids formed. A continuous Soxhlet extraction (4) however required a longer time and a higher tempera-
of this mixture over seven days gave pure 1.
ture compared to that in the synthesis of complex 5. The
Compound 1 was converted into its acyl chloride and purity of the complexes was checked by different analytical
this was reacted with l-tyrosine ethyl ester hydrochloride and spectroscopic methods and the analytical data matched
in acetonitrile in presence of triethylamine. Purification by well with those of the expected formulations.
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Interfacial Electron Transfer Dynamics of RuII Polypyridyl Based Sensitizers
Spectroscopic Properties – UV/Vis Absorption and
Photoluminescence Measurements
potential surface also contains a contribution from their
electron-donating ligands and nonradiative transitions are
less active compared to 7.
Figure 1 shows the optical absorption spectra of 5 and 6
in acetonitrile. The spectrum of 7 is included for compari-
son.[37] The low energy broad absorption band for 5 at
456 nm and that of 6 at 456 nm along with a shoulder at
433 nm arise due to overlapping metal-to-ligand charge
Electrochemical Studies
Cyclic voltammograms of 5 and 6 in acetonitrile were
very similar in nature (Figure 2). The RuII/III redox poten-
tial for 5 was 1.35 V and that of 6 was 1.30 V in degassed
acetonitrile. Both tyrosine and phenol show irreversible oxi-
dation peaks at 1.03 and 0.98 V in 5 and 6, respectively. The
irreversible nature of the oxidation peak has been pre-
viously reported for phenols, and is because the oxidized
species have lifetimes shorter than the timescale of the in-
strument.[42] The potential values quoted henceforth for
these electron-donating phenolic groups are therefore the
oxidation potentials and those for the metal centre are the
redox potentials.
II
transfer (MLCT) based dRuIIǞπ*2,2Ј–bpy, dRu Ǟπ*bpy–cat
and dRuII Ǟπ*bpy–tyr or dRuII Ǟπ*bpy–phenol transitions. The
absorption of tyrosine (λmax ≈ 274 nm, ε = 1405)[38] in 5 and
that for phenol in 6 (λmax at ca. 270 nm, ε = 2340)[39] are
masked due to strong intra- and/or interligand π–π* transi-
tions at 288 nm (ε ≈ 105),[40,41] and the band at 245 nm
along with the shoulder at 253 nm are assigned to higher
energy MLCT d–π* transitions.[41] The absorption bands at
354 and 322 nm for 5 and 6, respectively, can be attributed
to interligand charge transfer transitions from πbpy to
[41]
π*bpy–tyr and from πbpy to π*bpy–phenol
.
The relative shift
of 32 nm for 6 compared to 5 is due to the higher donating
ability of phenol (Eox = 0.98 V) compared to tyrosine (Eox
= 1.03 V) (see below for electrochemical data).
Figure 2. Cyclic voltammograms of (a) 5 and (b) 6 in acetonitrile.
A saturated calomel electrode (SCE) was used as the reference elec-
trode. The scan rate for both scans was 100 mV/s.
In order to comprehend the feasibility of electron injec-
tion into the conduction band of the semiconductor nano-
particles it becomes necessary to calculate the redox poten-
tial of the excited state of the dye. The E0–0 transition en-
ergy was calculated to be 2.21 and 2.24 eV for 5 and 6,
respectively, from the intersection points of the excitation
and emission spectra in acetonitrile. The excited state po-
tentials E(S+/S*)[43,44] were thus calculated to be –0.86 and
–0.94 V for 5 and 6, respectively, following the equation
[E(S+/S*)] = [E(S+/S)] – E0–0. The excited state potentials
being above the conduction band level, electron injection
from the excited state of the sensitizers into the conduction
band of TiO2 is feasible. As evident from the cyclic voltam-
mograms, the oxidation potential of the tyrosine and phen-
olic moieties in 5 and 6 are lower than the oxidation poten-
tial of the ruthenium centre. Therefore, electron transfer
from tyrosine or phenol to the RuII centre is thermodynam-
ically possible.
Figure 1. A: Absorption spectra of (a) 5, (b) 6 and (c) 7 in acetoni-
trile (c = 3ϫ 10–5 m); B: Emission spectra of (d) 5, (e) 6 and (f) 7
in acetonitrile. Emisson spectra were recorded for identical ab-
sorbances of the three complexes at an excitation wavelength of
457 nm.
Steady state emission spectra of 5, 6 and 7 in acetonitrile
were also recorded and these are shown in part B of Fig-
ure 1. In this study our main aim is to find the effect of
electron-donating groups in Ru complex-sensitized TiO2
nanoparticles after exciting the MLCT band. Therefore, we
have carried out steady state emission studies of 5 and 6
after excitation at 456 nm. Figure 1 (B) shows that 5 emits
with λmax at 635 nm, and 6 emits at 614 nm. The blueshift
observed in the emission spectrum of 6 compared to that
of 5 is due to the presence of the phenol moiety, which acts
as a better donor than tyrosine and raises the π*bpy lowest
unoccupied molecular orbital level. Interestingly, although
we observe the emission intensity of 5 and 6 to be similar,
the emission intensity of 7 (Figure 1, f) is much lower than
that of 5 and 6 in spite of the absence of an electron-donat-
ing group. This might be due to the fact that the excited
Dye Binding with TiO2 Nanoparticles
The aim of this investigation is to study IET on semicon-
state potential surface of 7 is dominated by the bpy–cate- ductor surfaces, therefore it is very important to study the
chol ligand where the de-excitation process is very fast due binding properties of the dye and nanoparticulate semicon-
to H-bond interactions between the solvent and the cate- ductor systems. The nature of the binding between a sensi-
chol moiety. However, in case of 5 and 6, the excited state tizer and a semiconductor is known to influence the excited
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state properties and IET behaviour.[4] Strong binding serves Excited State Dynamics of the Free Complexes in
to anchor the sensitizer in place and control interfacial elec- Acetonitrile
tronic coupling. This is also known to influence the redox
In order to develop a comprehensive understanding of
potentials of the sensitizers and the semiconductor nano-
the dynamics of electron injection into the semiconductor
particles. We have observed that RuII– and OsII–polypyridyl
nanoparticles, it is essential to study the excited state dy-
complexes with pendant catecholate moiety bind strongly
namics of the photoinduced processes associated with the
to TiO2 nanoparticles.[35–37,45,46] Figure 3 shows the optical
free sensitizer molecules. We have carried out femtosecond
absorption and emission spectra of 5 and 6 in aqueous solu-
transient absorption studies (in the visible and near IR re-
tion in the presence and absence of TiO2 nanoparticles. The
gion) of 5 and 6 in acetonitrile. Figures 4 and 5 show the
spectral features observed in water are very similar to those
transient absorption spectra of 5 and 6 in acetonitrile,
in acetonitrile (Figure 1). The absorption spectra show a
respectively.
prominent increase and broadening, along with a slight red-
shift, of the MLCT band at 456 nm in the presence of TiO2
nanoparticles for both complexes, which can be explained
by a strong interaction of the sensitizer molecules with TiO2
nanoparticles through the formation of a five-membered
ring.[47,48] This observation is in accordance with results re-
ported earlier where we have shown that dyes bound to the
TiO2 surface through catecholate anchoring groups interact
very strongly.[35,36,45,46,49–52]
Figure 4. Transient absorption spectra of 5 in acetonitrile at dif-
ferent delay times following excitation at 400 nm. Inset: kinetic
trace of the excited triplet state (3MLCT) of 5 at 650 nm in acetoni-
trile after excitation at 400 nm.
Figure 3. i) Absorption spectra of A: 5, B: 6, C: 5 in the presence
of TiO2 nanoparticles, D: 6 in the presence of TiO2 nanoparticles
and E: TiO2 nanoparticles; ii) emission spectra of a: 5, b: 6, c: 5 in
the presence of TiO2 nanoparticles and d: 6 in the presence of TiO2
nanoparticles. Dye concentration for all UV measurements is
2.5ϫ10–5 m, and 3ϫ10–5 m for the fluorescence measurements.
TiO2 concentration is 20 g/L for all measurements.
Figure 5. Transient absorption spectra of 6 in acetonitrile at dif-
ferent delay times following excitation at 400 nm. Inset: kinetic
trace of the excited triplet state (3MLCT) of 6 at 650 nm in water
after excitation at 400 nm.
To confirm the formation of the complex, we have con-
structed a Benesi–Hildebrand plot for 5 and 6 by recording
the optical absorption spectrum of the complexes with in-
creasing TiO2 concentrations. From the Benesi–Hildebrand ranging from 510–780 nm and the other in the near IR re-
plot, equilibrium constants of
5.33ϫ103 and gion ranging from 830–1000 nm along with a bleach before
4.28ϫ103 m–1 were determined for 5–TiO2 and 6–TiO2, 500 nm due to ground state MLCT absorption. These tran-
respectively. Molar extinction coefficients of sient absorption bands are assigned to excited triplet state
Both spectra show two broad absorption features, one
1.04ϫ104 m–1 cm–1 were determined for both 5–TiO2 and absorption,[35–37,46] because intersystem crossing in RuII–
6–TiO2. The strong interaction of the sensitizers with the polypyridyl complexes is known to be very fast and is com-
semiconductor nanoparticles is further supported by the pleted in 15Ϯ10 fs.[53] Thus, the singlet to triplet conversion
fact that the luminescence spectra of both the complexes is expected to have occurred within the pulse width of the
are quenched considerably in the presence of TiO2 nanopar- instrument (Ͻ 100fs). We monitored excited state dynamics
ticles due to electron injection (Figure 3). Thus, steady state of 5 and 6 by monitoring the kinetics at 650 nm, shown
studies confirm strong binding of the sensitizer molecules inset in Figure 4 and Figure 5, respectively.
with the nanoparticles and permit further studies into their
effectiveness as efficient sensitizers and the possible effects ponential growth with time constants of Ͻ 100 fs (85%)
of the electron-donating groups attached. and 10 ps (15%). The first growth component is pulse width
The kinetic trace for 5 at 650 nm was found to have biex-
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Interfacial Electron Transfer Dynamics of RuII Polypyridyl Based Sensitizers
limited (Ͻ 100 fs) and can be attributed to the formation
of a singlet and/or triplet state. The second growth compo-
nent can be attributed to a vibrational relaxation of the
3MLCT state.[37,54–56] It has also been observed that the ki-
netic trace does not decay on the 1 ns timescale. On the
other hand, the kinetic decay trace of 6 monitored at
650 nm shows single exponential growth (Ͻ 100 fs) followed
by biexponential decay with time constants of 30 ps
(15.4%) and Ͼ 1 ns (84.6%). As reported previously, the
lifetimes of the triplet MLCT state of RuII–polypyridyl
complexes are Ͼ 100 ns,[57] so we can safely attribute these
long components in both complexes to excited triplet states.
It is interesting to see that 6 (Figure 5, inset) has an extra
decay time constant of 30 ps in addition to the Ͼ 1 ns com-
ponent compared to 5, which has only one decay compo-
nent of Ͼ 1 ns. We have previously carried out ultrafast ex-
cited spectroscopic studies on a series of ruthenium com-
plexes[35,37,46] and observed only one long time decay com-
ponent (Ͼ 1 ns), except for Ru(CN)4(bpy–cat),[2–36] which
has an extra decay component of 10 ps. This component
was thought to be due to an extra decay channel associated
with the excited states. The cyano groups might form H-
bonds with the surrounding solvent molecules and so the
excited state energy could relax to the solvent through the
H-bonds. In a similar way, the hydroxy group of the bpy–
phenol moiety in 6 can form H-bonds with the solvent mo-
lecules and the excited state could relax resulting in the
30 ps decay component. The phenolic hydroxy group is in
Figure 6. Transient absorption spectra of 5–TiO2 in water at dif-
ferent time delays after excitation at 400 nm. Typical concentration
for 5 was ca. 200 μm and about 20 g/L for TiO2 nanoparticles. Inset:
kinetic trace of the radical cation of 5 (5·+) at 590 nm.
Figure 7. Transient absorption spectra of 6–TiO2 in water at dif-
ferent time delays after excitation at 400 nm. Typical concentration
direct conjugation with the chromophore centre in 6, unlike for 6 was ca. 200 μm and ca. 20 g/L for TiO2 nanoparticles. Inset:
kinetic trace of the radical cation of 6 (6·+) at 610 nm.
that in 5, which is linked by a nonconjugated amide spacer.
This could be the reason why the decay component is ab-
sent in 5.
The electron injection time can be predicted by monitor-
ing the time of appearance of the electron signal. It was
found to be pulse width limited (Ͻ 100 fs) and single ex-
ponential at all the wavelengths monitored and for both
compounds. The electron is injected in majority from the
nonthermalized states and not from the thermalized states,
Transient Absorption Measurements of 5 and 6 on a TiO2
Surface
In steady state emission studies we have observed that the confirmed by the fact that no associated growth for either
photoluminescece intensity of both 5 and 6 was drastically the radical cation signal or the electron signal was observed
reduced in the presence of TiO2 nanoparticles (Figure 3), and that the kinetic traces decay with almost similar time
which confirmed IET from the sensitizers to TiO2. To probe constants from 540–900 nm. Had there been injection from
the ET dynamics and the effect of the electron-donating the thermalized triplet states, we would have observed a
groups on a shorter timescale, we carried out femtosecond growth of the radical cation signal in the early timescale
transient absorption measurements on the dye–TiO2 sys- along with a decay of the triplet state.[64]
tems following excitation with a laser source of 400 nm.
In addition, our previous studies confirm that the tran-
Figures 6 and 7 show the transient absorption spectra sients obtained at different wavelengths are due to the
of 5–TiO2 and 6–TiO2, respectively. Both spectra show two charge separated species and that any contribution due to
broad absorption bands, one ranging from 540–750 nm and excited states is negligible.[35,36,46] The nonthermalized ex-
the other in the region of 800–1000 nm. The first absorp- cited states involved in IET are presumably both the hot
tion feature can be assigned to the formation of the radical singlet and triplet states because the ISC process is known
cation (5·+ or 6·+) based on our previous results.[35,37,46] The to be very fast in RuII–polypyridyl complexes and this com-
lower energy absorption band in the 800–1000 nm region is petes with the electron injection into TiO2. Complexes 5
assigned to electrons in the conduction band of the TiO2 and 6 are anchored to the TiO2 surface through the cate-
nanoparticles. This assignment is corroborated by the detec- cholate functionality resulting in very strong electronic cou-
tion of electrons in the conduction band by visible,[58,59,60] pling, which is responsible for electron injection from the
near IR[61] and mid IR[62,63] absorption. The kinetic decay nonthermalized states, hence a single exponential and ul-
trace of radical cations for both 5 and 6 are shown inset in trafast electron injection. The main aim of this investigation
Figures 6 and 7, respectively.
is to monitor the effect of the electron-donating centre on
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Scheme 3. Schematic representation of IET in 5–TiO2 and 7–TiO2. The effect of a secondary electron-donating group in the IET dynamics
of 5–TiO2 has been demonstrated.
ET dynamics. Provided that the charge separation at the τ1 = 1.5 ps (37.6%), τ2 = 70 ps (19.4%), τ3 Ͼ 1 ns (43%)
dye–semiconductor interface is maintained efficiently, the (Figure 8, c).
dye-sensitized solar cell can be made to work resourcefully.
It is interesting to see that the bleach recovery kinetics is
Charge recombination, i.e. BET dynamics, therefore be- slowest for 6–TiO2. We have also monitored injected elec-
comes necessary to be studied as it is one of the major path- tron in the conduction band at 900 nm to compare the BET
ways for loss of the effective electron concentration in the dynamics (Figure 9).
conduction band. The injected electron has to escape from
the reaction distance of recombination and reach the bulk
of the electrode by downhill migration for it to contribute
to the photocurrent response of the cell.[65] The dynamics
of the BET process can be determined by monitoring the
decay of either the conduction band electrons or the radical
cation or by monitoring the recovery of the ground state
bleach. Figure 8 (a and b) shows the bleach recovery kinet-
ics of 5–TiO2 and 6–TiO2, respectively. In our earlier re-
ports we discussed IET dynamics in 7–TiO2 where no elec-
tron-donating moiety was present (Scheme 3). In order to
find the effect of the electron-donating groups, it is impor-
tant to compare the BET dynamics of all the systems, (5-,
6- and 7-sensitized TiO2 nanoparticles). We compare the
bleach recovery kinetics for these three systems in Figure 8.
Figure 9. Transient decay kinetics of injected electron at 900 nm in
a) 5–TiO2, b) 6–TiO2 and c) 7–TiO2 after excitation at 400 nm.
The kinetic traces of the injected electron in 5–TiO2 (Fig-
ure 9, a) and 6–TiO2 (Figure 9, b) can be fitted multiex-
ponentially with time constants τ1 = 3.5 ps (28%), τ2
=
70 ps (28%) and τ3 Ͼ 1 ns (44%) and τ1 = 3.3 ps (25.8%),
τ2 = 110 ps (29%) and τ3 Ͼ 1 ns (45.2%), respectively. For
7–TiO2 (Figure 9, c) kinetics at 900 nm[37] can be fitted with
time constants τ1 = 1.5 ps (34.8%), τ2 = 70 ps (26.8%) and
τ3 Ͼ 1 ns (38.4%). As is evident from the above time con-
stants, the BET was found to be faster in these systems
than that of dyes with carboxylate anchoring groups.[34,66]
Stronger binding of the catecholate functionality is respon-
sible for the fast BET in our systems.
Figure 8. Bleach recovery kinetics at 480 nm in a) 5–TiO2, b) 6–
TiO2 and c) 7–TiO2 system after excitation at 400 nm.
Effect of Electron-donating Group in the IET Reaction
The kinetic traces of 5–TiO2 (Figure 8, a) and 6–TiO2
(Figure 8, b) can be fitted multiexponentially with time con-
A good deal of research has been devoted to searching
stants τ1 = 2.7 ps (39.5%), τ2 = 70 ps (16%) and τ3 Ͼ 1 ns for ways to slow the charge recombination process between
(44.5%) and τ1 = 3.3 ps (19%), τ2 = 110 ps (32%) and τ3 Ͼ the photo-oxidized dye and the conduction band electrons
1 ns (49%), respectively. We have already reported the ble- in dye-sensitized TiO2 nanoparticles. While it is possible to
ach recovery kinetics for 7–TiO2 fitted with time constants position the sensitizer core away from the nanoparticle sur-
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Interfacial Electron Transfer Dynamics of RuII Polypyridyl Based Sensitizers
face, a fast electron injection rate has to be compromised the overall free energy of the reaction, provided that
at the expense of slow charge recombination.[67,68] In an- the effect of electron-donating group is absent. ΔG0 = EC
–
other approach, people have tried introducing an additional ES/S+, where EC (–0.49 V)[74] is the potential of the electrons
+
electron donor so that the initial electron injection process in the conduction band of the semiconductor and ES/S is
is not affected. Several molecular dyads have been studied the redox potential of the dye. Using this equation the
in this context.[19–21,23–29,69,70] Phenolic donor groups have (–ΔG0) values for 5–TiO2, 6–TiO2 and 7–TiO2 were calcu-
been explored as an electron-donating functionality, in par- lated to be 1.84 eV, 1.79 eV and 1.81 eV, respectively. Inter-
ticular by Sundstrom et al.[30,31,71] Most of these studies estingly, the free energy of reaction (–ΔG0) is very similar
have been carried out on the nano- and microsecond times- for all three systems, although we observe very different ET
cales. To understand the influence of the electron-donating dynamics. It has been demonstrated previously that the
moiety it is important to study and compare the ET dynam- BET reaction in dye–TiO2 systems falls into the Marcus
ics for 5, 6 and 7 on the ultrafast timescale. In this report, inverted regime.[75–77] This means that as we increase the
the electron transfer processes were initiated following a la- free energy of reaction (–ΔG0), the rate of reaction will de-
ser pulse of 400 nm, which populated the MLCT states and crease. In this study, if there was no effect of the electron-
from which electrons are injected into the conduction band donating groups on the ET dynamics then we would have
of TiO2. Immediately after electron injection, a hole will seen the fastest ET dynamics in 6–TiO2 as the free energy
be localized in the electron-donating groups (tyrosine and of the reaction (–ΔG0) is the lowest (1.79 eV). Therefore, we
phenol) and as a result there will be an increase in spatial have clearly demonstrated the effect of the electron-donat-
charge separation because the electron will be localized in ing groups on the ET dynamics. Based on our data and
TiO2, and we expect that the BET dynamics will be slower. observations, Scheme 3 is proposed for photoinduced elec-
We have shown the bleach recovery kinetics in Figure 8 tron transfer processes for 5–TiO2 and 6–TiO2
for different systems, and it is clear that the charge recombi-
After light excitation of the ruthenium chromophore,
nation reaction is fastest in 7–TiO2 (where 7 does not have electron injection occurs from the MLCT excited state to
a secondary electron-donating group). We have also ob- TiO2, producing the charge-separated state of RuIII and
served a similar trend as we have monitored the kinetic de- electrons in the conduction band of TiO2. The hole may
cay trace for the injected electron (Figure 9). From Fig- now become localized in the secondary electron-donating
ures 8 and 9 it is clear that the electron-donating groups groups and thus coupling between the electron in the con-
play a major role in charge recombination reactions in dye– duction band and the hole in the sensitizer gets reduced.
TiO2 nanoparticle systems. Phenol is a better electron do- This might be responsible for the slow BET observed for 5–
nor than tyrosine (as observed in electrochemical studies) TiO2 and 6–TiO2 in the early timescale. It would have been
and so, in the early timescale, the bleach recovery of 6–TiO2 best, had we been able to determine the rate of electron
is the slowest, followed by that of 5–TiO2, and the recovery transfer from tyrosine or phenol to RuIII by monitoring the
of 7–TiO2 is the fastest. To confirm that the electron-donat- respective oxidized radicals (tyrosyl and phenoxy) in the
ing group is indeed taking part in the electron transfer dy- femtosecond transient absorption spectrum. We are de-
namics, it is important to monitor the oxidized donor moi- veloping new RuII–polypyridyl-based sensitizer dyes with
ety (tyrosyl and phenoxy radical cations in 5 and 6, respec- covalently attached electron-donating groups such that the
tively) in the transient spectra. However, these radicals ab- oxidized donor group can be monitored directly in the vis-
sorb below 450 nm in the transient spectra,[71,72] which is ible region (above 450 nm) so that the ET rate constants
beyond the detection range of our femtosecond transient can be determined without ambiguity.
spectrometer (450–1000 nm). To reconfirm the influence of
the electron-donating centers on the ET dynamics, we have
determined the free energy of the respective reactions in our
Conclusions
systems. Different BET rates for 5, 6 and 7 can be explained
by Marcus semiclassical electron transfer theory.[73] Accord-
Two new RuII–polypyridyl complexes 5 and 6, with pen-
ing to this theory, the BET rate constant (kBET) is expressed dant catecholate functionality and tyrosine or phenol as
as
electron-donating groups, have been synthesized and char-
acterized. The catecholate functionality allows strong cou-
pling of these sensitizer molecules with nanoparticulate
TiO2. We have investigated IET dynamics in 5–TiO2 and 6–
TiO2 using ultrafast transient absorption spectroscopy. On
where HAB is the coupling element, Λ is the reorganization excitation with 400 nm laser pulse, a transient absorption
energy and ΔG0 is the overall free energy of the reaction. band for the dye radical cation (D·+) and a broad absorp-
We can safely assume that the electronic coupling in the tion band for the conduction band electrons are observed.
dye–nanoparticle systems is nearly the same as for 5–TiO2, Electron injection was found to be single exponential and
6–TiO2 and 7–TiO2 systems. The reorganization energy can pulse width limited (Ͻ 100 fs) indicating electron injection
also be assumed to be similar considering that all of the from a nonthermalized singlet state (1MLCT), and/or trip-
studies have been performed under identical conditions. let state (3MLCT). BET dynamics were studied by monitor-
Therefore, the BET rate for these systems will depend on ing the decay kinetics of the injected electron in the conduc-
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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4193

T. Banerjee, S. Rawalekar, A. Das, H. N. Ghosh
FULL PAPER
sizes of the pump beam and probe beam at the crossing point
around 500 and 300 microns, respectively. The excitation energy
density (at both 800 and 400 nm) was adjusted to ca. 2500 μJ/cm2.
The noise level of the white light is about about 0.5% with occa-
sional spikes due to oscillator fluctuation. We have noticed that
most laser noise is low-frequency noise and can be eliminated by
comparing the adjacent probe laser pulses (pump blocked vs. un-
blocked using a mechanical chopper). The typical noise in the mea-
sured absorbance change is about Ͻ 0.3%. The instrument re-
sponse function for 400 nm excitation was obtained by fitting the
tion band of TiO2 and by the recovery of the ground state
bleach. BET dynamics for 5 and 6 were compared with that
of previously reported 7. The BET rate was found to be
slowest in 6–TiO2 and fastest in 7–TiO2 on an early times-
cale. On laser excitation, electrons are injected from the
MLCT state into the conduction band of TiO2. Electro-
chemical studies suggest that phenol is a better donating
group than tyrosine, which corroborates the decreased BET
rate observed for 6 compared to 5. However, the BET rate
for 7 is the fastest due to the absence of electron-donating rise time of the bleach of a sodium salt of meso-tetrakis(4-sulfon-
atophenyl)porphyrin at 710 nm and found to be 120 fs.
groups.
Preparation of Sample Solutions: The ruthenium complexes are in-
soluble in water and so sensitization was performed by dissolving
the complexes in the smallest possible volume of acetonitrile (less
than 2% of the total volume) and then adding the dissolved dye
into the aqueous colloidal solution of the nanoparticles. The re-
sulting solutions were stirred for half an hour and then kept in
dark for 5 h for the dye to covalently bind to TiO2. For all the
measurements the sample solutions were degassed by continuously
bubbling high purity nitrogen through the solutions.
Experimental Section
Materials: Titanium(IV) tetraisopropoxide (97%), isopropyl
alcohol, 4,4Ј-dimethyl-2,2Ј-bipyridine, n-butyllithium, 3,4-dimeth-
oxybenzaldehyde, 2,2Ј-bipyridine, 4-methoxybenzaldehyde, 2-ace-
tylpyridine, l-tyrosine ethyl ester hydrochloride and isopropyl
alcohol were obtained from Sigma–Aldrich and used as received.
Solvents such as THF, acetonitrile and isopropyl alcohol were dried
and distilled prior to use. Nanopure water (Barnsted System, USA)
was used in the preparation of aqueous solutions. All other rea-
gents (AR grade) were procured from S.D. Fine Chemicals (India).
HPLC grade acetonitrile (E. Merck, Mumbai, India) was used for
all spectrophotometric titrations. Solvents were degassed thor-
oughly with IOLAR grade dinitrogen gas before use in the prepara-
tion of standard solutions.
Synthesis of TiO2 Nanoparticles: TiO2 nanoparticles were prepared
by controlled hydrolysis of titanium(IV) tetraisopropoxide.[60,81]
A
solution of Ti[OCH(CH3)2]4 (5 mL, Aldrich, 97%) in isopropyl
alcohol (95 mL) was added dropwise (1 mL/min) to nanopure
water (900 mL) at 2 °C at pH 1.5 (adjusted with HNO3). The solu-
tion was continuously stirred for 10–12 h until the formation of a
transparent colloid. The colloidal solution was concentrated at 35–
40 °C with a rotary evaporator and dried with a nitrogen stream to
yield a white powder. In this work, all colloidal samples have been
prepared after dispersing the dry TiO2 nanoparticles in water
(20 g/L).
Analytical Methods: FTIR spectra were recorded as KBr pellets in
a cell fitted with a KBr window, using a Perkin–Elmer Spectra GX
1
2000 spectrometer. H NMR spectra were recorded with a Bruker
200 MHz FT NMR (Avance-DPX 200) or Bruker 500 MHz FT
NMR (Avance-DPX 500) using CD3CN as the solvent and tet-
ramethylsilane as an internal standard. ESI-MS measurements
were carried out with a Waters QTof-Micro instrument. Micro-
analyses (C, H, N) were performed using a Perkin–Elmer 4100 ele-
mental analyzer. Electronic spectra were recorded with a Shimadzu
UV-3101 PC spectrophotometer; room temperature luminescence
spectra were recorded with either a Fluorolog (Horiba Jobinyvon)
or Perkin–Elmer LS 50B luminescence spectrofluorimeter, fitted
with a red-sensitive photomultiplier tube. Electrochemical experi-
ments were performed in acetonitrile with a CH-660A electrochem-
ical instrument with a conventional three-electrode cell assembly
and SCE as the reference electrode. All potentials have been cor-
rected and quoted with respect to the Normal Hydrgen Electrode
(NHE) in water.[78]
Syntheses
4Ј-Methyl-2,2Ј-bipyridinyl-4-carboxylic Acid (1): Compound 1 was
prepared according to an adaptation of a reported procedure.[82]
4,4Ј-Dimethyl-2,2Ј-bipyridine (3 g, 16.3 mmol) and SeO2 (2.2 g,
19.8 mmol) were dissolved in 1,4-dioxane (120 mL) and heated to
reflux for 30 h with continuous stirring. The solution was filtered
hot through celite. The filtrate was evaporated to dryness and was
suspended in ethanol. To this suspension was added aq. AgNO3
(3.04 g, 17.9 mmol) and a colour change to reddish yellow was ob-
served. The suspension was stirred rapidly and NaOH solution
(75 mL, 1 m) was added dropwise over half an hour. The reaction
mixture was stirred for 24 h. Ethanol was removed by evaporation
and the residue was collected by filtration in a grade 4 sintered
glass crucible and washed with dilute aqueous NaOH and water.
The filtrate was extracted into dichloromethane, and the pH of the
aqueous layer was adjusted to 3.5 by addition of 6 n hydrochloric
acid to precipitate a white solid. The solution along with the pre-
cipitate was kept in refrigerator for 6 h and then collected by fil-
tration and dried. Pure 1 was obtained after continuously ex-
tracting this solid with acetone in a Soxhlet apparatus for seven
days; yield 2.69 g, 77%. ESI-MS (m/z): calculated for C12H10N2O2
214.22; found 215.15 [M + H+]. 1H NMR (500 MHz, [D6]DMSO):
δ = 8.87 [d, J = 4.5 Hz, 1 H, H6 (bpy)]; 8.82 [s, 1 H, H3 (bpy)];
8.58 [d, J = 4.5 Hz, 1 H, H6Ј (bpy)]; 8.27 [s, 1 H, H3Ј (bpy)]; 7.87
[d, J = 4.5 Hz, 1 H, H5 (bpy)]; 7.34 [d, J = 4 Hz, 1 H, H5Ј (bpy)];
Femtosecond Visible Spectrometer: The femtosecond tunable visible
spectrometer was developed based on a multi-pass amplified femto-
second Ti:Sapphire laser system from Avesta, Russia (1 kHz repeti-
tion rate at 800 nm, 50 fs, 800 μJ/pulse) as described pre-
viously.[79,80] The 800 nm output pulse from the multi-pass ampli-
fier is split into two parts to generate pump and probe pulses. In
this investigation we have used both the 800 nm (fundamental) and
the frequency doubled 400 nm pulses as excitation sources. To gen-
erate pump pulses at 400 nm one part of 800 nm with 200 μJ/pulse
is frequency doubled in BBO crystals. To generate visible probe
pulses about 3 μJ of the 800 nm beam is focused onto a 1.5 mm
thick sapphire window. The intensity of the 800 nm beam is ad-
justed by iris size and ND filters to obtain a stable white light
continuum in the 400 to over 1000 nm region. The probe pulses are
split into the signal and reference beams and are detected by two
matched photodiodes with variable gain. We have kept the spot
2.44 (s, 3 H, CH ) ppm. IR (KBr): ν = 3427 (OH), 1710 (C=O)
˜
3
cm–1. C12H12N2O2 (216.24): calcd. C 67.28, H 4.71, N 13.08; found
C 67.0, H 4.97, N 13.1.
Ethyl-3-(4-hydroxyphenyl)-2-[(4Ј-methyl-2,2Ј-bipyridinyl-4-carbonyl)-
amino]propionate (2): Synthesis of 2 was performed following a
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Interfacial Electron Transfer Dynamics of RuII Polypyridyl Based Sensitizers
modification of a literature procedure.[71] Compound 1 (500 mg,
2.336 mmol) was added to thionyl chloride (25 mL) and the solu-
tion was heated to reflux for 6 h. Excess thionyl chloride was re-
moved by downward distillation and then under reduced pressure.
To a solution of l-tyrosine ethyl ester hydrochloride (636.7 mg,
2.562 mmol) in dry distilled acetonitrile (30 mL) was added dry
triethylamine (1 mL) and the acid chloride dissolved in dry acetoni-
trile dropwise over one hour. The solution was stirred for half an
hour at room temperature and then heated to reflux for 12 h under
dinitrogen. The solution was then cooled and filtered. The filtrate
was evaporated to dryness, and the residue was dissolved in dichlo-
romethane (≈ 30 mL) and washed three times with water to remove
excess triethylamine. The organic phase was dried with anhydrous
sodium sulfate and the solvents evaporated to dryness. The crude
product was purified by chromatography using silica as the station-
ary phase and ethyl acetate/hexane as the eluent; yield 476.35 mg,
water to remove the excess KPF6. The crude product was purified
by column chromatography by using silica as the stationary phase
and acetonitrile/water/saturated aqueous KPF6 as the eluent. The
first fraction was collected and the solvents evaporated to dryness.
It was redissolved in dichloromethane and two drops of acetonitrile
and solvent extraction was repeated to remove the excess KPF6
used in the eluent. The organic phase was dried with anhydrous
sodium sulfate and evaporated to dryness to give the desired prod-
uct. The compound was further purified by the vapour diffusion
method of recrystallization (acetonitrile/diethyl ether). Although
single crystals were not obtained, pure 5 was deposited; yield
85 mg, 27.4%. ESI-MS (m/z): calculated for C52H47N7O6Ru, [M –
2PF6 ] 967.04; found 966.3 [M – 2PF6– – H+]. 1H NMR (200 MHz,
–
CD3CN): δ = 8.68 [s, 1 H, H6 (bpy–tyr)]; 8.49 [d, J = 8.2 Hz, 5 H,
H6 and H6Ј (bpy), H6 and H6Ј (bpy–cat), H6Ј (bpy–tyr)]; 8.05 [m, 3
H, H3 and H3Ј (bpy), H3 (bpy–tyr)]; 7.81 [m, 2 H, H3Ј (bpy–tyr),
H3Ј (bpy–cat)]; 7.73–7.71 [m, 3 H, H4 and H4Ј (bpy), H3 (bpy–cat)];
50%. ESI-MS (m/z): calculated for C23H23N3O4 405.45; found
1
406.38 [M + H+]. H NMR (200 MHz, CDCl3): δ = 8.71 [d, J = 7.62–7.54 [m, 4 H, H (ethenyl), H5 (bpy–tyr), H5 and H5Ј (bpy)];
5 Hz, 1 H, H6 (bpy)]; 8.55 [s, 1 H, H3 (bpy)]; 8.48 [d, J = 4.8 Hz,
1 H, H6Ј (bpy)]; 8.12 [s, 1 H, H3Ј (bpy)]; 7.64 [d, J = 4.8 Hz, 1 H,
7.4 [s, 3 H, H5Ј (bpy–tyr), H5Ј (bpy–cat), H6 (catechol)]; 7.28–7.25
[m, 2 H, H2 and H5 (catechol)]; 7.15–7.04 [m, 4 H, H (ethenyl), H5
H5 (bpy)]; 7.25 [d, J = 4.2 Hz, 1 H, H5Ј (bpy)]; 7.1 [d, J = 8.4 Hz, (bpy–cat), H2 and H6 (phenol)]; 6.82 [d, J = 6.8 Hz, 2 H, H3 and
2 H, H3 (phenyl) and H5 (phenyl)]; 6.72 [d, J = 8.2 Hz, 2 H, H2
(phenyl) and H6 (phenyl)]; 4.86–4.78 [m, 1 H, CH(COOEt)]; 4.18
(q, J = 7.2 Hz, 2 H, COOCH2CH3); 3.26–2.98 (m, 2 H, CH2–ph);
2.43 (s, 3 H, bpy–CH3); 1.23 (t, J = 7.2 Hz, 3 H, COOCH2CH3)
H5 (phenol)]; 4.85–4.74 [m, 1 H, CH(COOEt)]; 4.15 (q, J = 7 Hz,
2 H, COOCH2CH3); 3.17–3.03 (m, 2 H, CH2–phenol); 2.56 [s, 6
H, CH3 (bpy–tyr), CH3 (bpy–cat)]; 1.12 (t, J = 7.2 Hz, 3 H, CO-
OCH CH ) ppm. IR (KBr): ν = 3419 (OH), 1734 (C=O) , 1605
˜
2
3
.
–
ester
ppm. IR (KBr): ν = 3320 (OH), 1728 (C=O) , 1656 (C=O)amide (C=C), 841 (PF6 ) cm–1. C52H47F6N7O6PRu (1112.02): calcd. C
˜
ester
cm–1. C23H23N3O4 (405.45): calcd. C 68.13, H 5.72, N 10.36; found 56.16, H 4.26, N 8.82; found C 55.9, H 4.3, N 8.6.
C 68.1, H 5.8, N 10.5.
Ruthenium(II) Complex 6: Compound 6 was synthesized in an anal-
ogous manner to that of 5. RuCl3·xH2O (51.2 mg, 0.247 mmol) and
2,2Ј-bipyridine (39 mg, 0.247 mmol) were dissolved in dimethyl-
formamide (30 mL) and heated to 80 °C for 4 h. To the resulting
solution was added 3 (61.5 mg, 0.247 mmol) and the reaction mix-
ture was heated to 110 °C for 5 h. The reaction mixture was allowed
to stand for 4 h before the addition of 4 (75.1 mg, 0.247 mmol) and
the reaction mixture was heated to 140 °C for 15 h. The purifica-
tion steps are the same as described above for 5; yield 90 mg, 33%.
4-(2,2Ј-Bipyridinyl-4-yl)phenol (3): Compound 3 was prepared fol-
lowing a reported procedure.[83] ESI-MS (m/z): calculated for
C16H12N2O 248.28, found 249.16 [M + H+], 271.15 [M + Na]+. 1H
NMR (200 MHz, [D6]DMSO): δ = 9.91 [s, 1 H, H (hydroxy)]; 8.73
[m, 1 H, H6Ј (bpy)]; 8.68 [d, J = 5.2 Hz, 1 H, H6 (bpy)]; 8.62 [d, J
= 1.2 Hz, 1 H, H3 (bpy)]; 8.44 [d, J = 7.8 Hz, 1 H, H3Ј (bpy)]; 7.98
[td, J = 7.7 and 2 Hz, 1 H, H4Ј (bpy)]; 7.73–7.70 [m, 3 H, H5 (bpy),
H2 (phenyl) and H6 (phenyl)]; 7.53–7.46 [m, 1 H, H5Ј (bpy)]; 6.94
[d, J = 8.6 Hz, 2 H, H3 (phenyl) and H5 (phenyl)] ppm. IR (KBr):
–
ESI-MS (m/z): calculated for C45H36N6O3Ru, [M – 2PF6 ] 809.88,
–
1
found 808.24 [M – 2PF6 – H+]. H NMR (200 MHz, CD3CN): δ
= 8.70 [s, 4 H, H3, H6 and H6Ј (bpy–ph), H6 or H6Ј (bpy)]; 8.53 [d,
J = 7.4 Hz, 4 H, H6 and H6Ј (bpy–cat), H6 or H6Ј (bpy), H5 or H5Ј
(bpy)]; 8.39 [d, J = 7.6 Hz, 1 H, H3Ј (bpy–ph)]; 8.12–8.04 [m, 5 H,
H3 and H3Ј (bpy), H3 and H3Ј (bpy–cat), H4Ј (bpy–ph)]; 7.88 [d, J
= 6 Hz, 2 H, H4 and H4Ј (bpy)]; 7.83–7.74 [m, 5 H, H2 and H6
(phenol), H5 (bpy–ph), H5 (bpy–cat), H5 or H5Ј (bpy)]; 7.62 [m, 2
H, H6 (catechol), H5 (bpy–cat)]; 7.45–7.39 [m, 3 H, H (ethenyl), H2
and H5 (catechol)]; 7.28 [t, J = 6.4 Hz, 1 H, H5Ј (bpy–ph)]; 7.02
ν = 3397 (OH) cm–1. C H N O (248.28): calcd. C 77.40, H 4.87,
˜
16 12
2
N 11.28; found C 77.2, H 5.0, N 11.5.
4-[2-(4Ј-Methyl-[2,2Ј]bipyridinyl-4-yl)vinyl]benzene-1,2-diol
(4):
Compound 4 was prepared following a literature procedure.[84] ESI-
MS (m/z): calculated for C19H16N2O2 304.34, found 305.22 [M +
1
H+]. H NMR (500 MHz, CD3OD): δ = 8.54 [d, J = 5.5 Hz, 1 H,
H6Ј (bpy)]; 8.52 [d, J = 5 Hz, 1 H, H6 (bpy)]; 8.34 [s, 1 H, H3Ј
(bpy)]; 8.13 [s, 1 H, H3 (bpy)]; 7.53 [dd, J = 4, 1 Hz, 1 H, H5Ј (bpy)];
7.43 [d, J = 16 Hz, 1 H, H (ethenyl)]; 7.32 [dd, J = 4, 1 Hz, 1 H,
H5 (bpy)]; 7.11 [d, J = 1.5 Hz, 1 H, H2 (phenyl)]; 7.02–6.98 [m, 2
H, H (ethenyl) and H6 (phenyl)]; 6.79 [d, J = 8 Hz, 1 H, H5
[m, 3 H, H (ethenyl), H3 and H5 (phenol)]; 2.55 [s, 3 H, CH3 (bpy–
–
cat)] ppm. IR (KBr): ν = 3438 (OH), 1606 (C=C), 840 (PF ) cm–1.
˜
6
C45H36F6N6O3PRu (954.85): calcd. C 56.6, H 3.8, N 8.8; found C
56.2, H 4.1, N 9.0.
(phenyl)]; 2.49 (s, 3 H, bpy-CH ) ppm IR (KBr): ν = 3238 (OH),
˜
3
.
1590 (C=C) cm–1. C19H16N2O2 (304.35): calcd. C 74.98, H 5.30, N
9.20; found C 74.5, H 5.3, N 9.7.
Acknowledgments
Ruthenium(II) Complex 5: RuCl3·xH2O (51.2 mg, 0.247 mmol) and
2,2Ј-bipyridine (39 mg, 0.247 mmol) were dissolved in dimethyl-
formamide (30 mL) and heated to 80 °C for 4 h. To the resulting
solution was added 2 (100 mg, 0.247 mmol) and the reaction mix-
The Board of Research in Nuclear Sciences (Department of Atomic
Energy, Government of India) and the Council of Scientific and
Industrial Research (CSIR) (India) have supported this work. T. B.
acknowledges BRNS for
Dr. P. K. Ghosh (CSMCRI), Dr. S. K. Sarkar (BARC) and Dr.
T. Mukherjee (BARC) for their interest in this work.
ture was heated to 110 °C for 4 h. After 4 h,
4 (75.1 mg,
a Research Fellowship. We thank
0.247 mmol) was added and the reaction mixture was heated to
135 °C for 8 h. The reaction mixture was allowed to cool to room
temperature before the solvent was evaporated. The residue was
dissolved in acetonitrile and was stirred with ten equiv. potassium
hexafluorophosphate for 24 h. Acetonitrile was evaporated and the
residue obtained was washed repeatedly with large volumes of
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Eur. J. Inorg. Chem. 2011, 4187–4197
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPER
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